A typical application of the present invention is in the art of embossing or cutting and scoring paper or similar material, wherein the work is accomplished by passing the material through the nip of a pair of rotating cylinders. In an earlier stage of the art, the design to be embossed, cut or scored was engraved on the surface of one of the cylinders itself, and when that engraved die cylinder became worn, it had to be discarded and replaced. Such an engraved die cylinder is very expensive, and therefore efforts have been directed to the provision of plate-like dies that can be attached to a cylinder in the manner of a printing plate so as to be replaceable without costly discard of the whole cylinder. In the current state of the art, the die is formed as a thin but sturdy flexible plate which is magnetically secured to the die cylinder.
Heretofore, however, attempts to employ such magnetic die plates have not been completely satisfactory, owing to limitations in the apparatus in which the die plates were used.
Conventionally, on each end of each of the cooperating cylinders there was fixed an accurately concentric bearer ring which had a somewhat larger radius than the cylinder proper. The cylinders were urged towards one another under substantial force whereby the bearer rings at adjacent ends of the cylinders were maintained in rolling engagement, and the distance between the surfaces of the two cylinders was thus fixed in dependence upon the differences in radius between the bearer rings and their respective cylinders. Each bearer ring was further engaged by a pair of rollers that were circumferentially spaced from one another and from the point of contact with the other bearer ring, and the forces that maintained the bearer rings engaged with one another were applied to them through these rollers. Each bearer ring, by its engagement with the rollers and the other bearer ring, was thus securely confined against radial motion.
The distance between the die cylinder and its cooperating anvil cylinder was intended to accommodate the thickness of the plate-like die carried by the die cylinder, and the fact that this distance was fixed gave rise to very troublesome problems in the use of such dies. In practice it is nearly impossible to produce a die plate with a thickness accurately matched to the cylinder-to-cylinder distance maintained by rollingly engaged bearer rings. Even if most of the die plate has the desired thickness, there is almost always some variation in thickness across the area of the plate. Although die plate thickness deviations are seldom very large, they were troublesome when the distance between cylinders was fixed. Portions of the die that were too thick were subject to relatively rapid wear, whereas portions that were too thin could not get close enough to the anvil cylinder to perform cutting and scoring functions effectively.
It was apparent that adjustability of the distance between cylinders would enable the cylinders to be spaced apart by whatever distance would best accommodate the thickness peculiarities of the particular die plate being used. If the cylinders could be adjusted to accommodate the thinnest portions of the die, the thickness of other portions would be reduced with use, bringing the die to a substantially uniform thickness. Furthermore, if the distance between the cylinders could be slightly reduced from time to time to compensate for wear on the die, the useful life of the die could be greatly prolonged and could be expected to exceed by a substantial amount the useful life of an engraved cylinder.
Heretofore, however, it has not been obvious how such adjustability of the distance between cylinders could be obtained while still providing for adequate support of the high radial forces imposed upon the cylinders.
It must be kept in mind that if the distance between the cylinders is adjustable, the adjusted distance between them must be accurately maintained while the cylinders are in rotation, and this requires that the cylinders be normally confined to rotation on rigidly fixed, accurately concentric axes. Since there is almost invariably a certain amount of eccentricity in a shaft and bearing support for a rotating cylinder, an adjustable cylinder-to-cylinder distance can not be accurately maintained by an attempted control of the positions of shaft bearings. Furthermore, radial forces that would be imposed upon the shaft bearings of embossing or die cutting cylinders would be likely to load those bearings to or above their rated capacities.
Thus the need for supporting relatively high radial forces on the cylinders while ensuring their accurately concentric rotation about axes that are normally fixed in relation to one another has heretofore appeared to be incompatible with adjustability of the distance between their axes so that the distance between the cylinders themselves can be brought to a desired value.